An investigation of the photoluminescence properties of divalent thulium generated by X-ray irradiation (X-irradiation) of nanocrystalline BaFCl:Tm3+ (250 ± 50 ppm) is reported. The X-irradiated samples show typical Tm2+ f–f luminescence with an excited state lifetime τ = 0.98 ± 0.05 ms in the near-infrared (∼1140 nm), but no d–f luminescence is observed for the BaFCl host material within the visible range. The Tm2+ ions are relatively stable under dim room light but are photoionized rapidly by sunlight or by fluorescent tube lighting. To study the X-ray storage mechanism, photoluminescence intensities of both Tm3+ and Tm2+ transitions were measured, and the reverse photoionization of Tm2+ was investigated as a function of the laser power density and the initial radiation dose. The photoionization data was then modeled by employing equations based on dispersive first order kinetics using a standard Γ distribution function for the separation between the reduced thulium ions and the hole traps. In accord with previous reports, the hole traps are most likely oxide impurities such as O2–, and the average of the separation is found to be ∼6–7 Å, i.e., a few interionic spacings.
Zhang, J., & Riesen, H. (2017). Controlled generation of Tm2+ Ions in nanocrystalline BaFCl:Tm3+ by X-ray irradiation. Journal of Physical Chemistry A, 121(4), 803-809. https://doi.org/10.1021/acs.jpca.6b12035